Coulomb stress changes and secondary stress triggering during the 2003 (Mw 6.6) Bam (Iran) earthquake

Abstract

The 26 December 2003 (Mw 6.6) Bam earthquake is one of the most destructive events in the recorded history of Iran. Good variable-slip fault models and precise aftershock data enabled us to calculate Coulomb stress changes on the surrounding faults, optimally oriented faults, and the aftershock nodal planes to learn whether they were brought closer to failure. We also test the role of secondary stress on the aftershocks triggering. Calculated stress on the surrounding faults showed that the southern part of the Gowk fault and middle part of the Dalfard and Bam faults received positive stress changes. Stress also increases at the ends of the ruptured fault plane to expand it. Our calculations showed that deeper part of the Bam fault planes received positive stress changes due to slipping on the shallow depth. It can lead to the occurrence of aftershocks and enhance the probability of large events in the future that represent a substantial remaining seismic hazard to the reconstructed city of Bam.By imparting stress on optimally oriented faults, the correlation between Coulomb stress changes and aftershock distribution is investigated. Obtained results showed good correlation between coulomb stress change and seismicity. From 331 well located aftershock hypocenters, about 270 events (>81%) are located in the positive stress changes area. The imparted stress due to the main shock and secondary aftershocks on the nodal planes of the 158 aftershocks is calculated. The results show that the imparted stress due to main shock encouraged about 60.1% of nodal planes and 77.8% of the chosen nodal planes of aftershocks and added secondary aftershock encouraged about 81.6% of chosen nodal planes. In other word by adding secondary triggering stress Coulomb Index for chosen nodal planes increased from 0.778 to 0.816 and about 80% of aftershocks receive greater positive stress changes.